There have been many attempts at making heated mattresses and heated mattress overlays for therapeutic patient warming. Therapeutic patient warming is especially important for patients during surgery. It is well known that without therapeutic intra-operative warming, most anesthetized surgical patients will become clinically hypothermic during surgery. Hypothermia has been linked to increased wound infections, increased blood loss, increased cardiac morbidity, prolonged ICU time, prolonged hospital stays, increased cost of surgery and increased death rates.
Since the early 1990s, the standard of care for surgical warming has been forced air warming blankets. Prior to that time, warm water mattresses were commonly used. The warm water mattresses went out of common use because they were relatively stiff and inflexible. The stiff water mattress negated any pressure relief that the underlaying support mattress may have provided. As a result, the combination of pressure applied to the boney prominences and the heat from the warm water mattress both reduced blood flow and accelerated metabolism, causing accelerated ischemic pressure injuries to the skin (“bed sores”). Additionally, the warmed water recirculating in the warming system was well known to be grossly contaminated with bacteria, which was especially important when a leak occurred. As a result, warm water mattresses are rarely used today.
Historically, electrically heated pads and blankets for the consumer market have been made with resistive wire heaters. Wire-based heaters have been questionably safe in consumer use. However, in the operating room environment with anesthetized patients, hot spots caused by the wires in normal use and the failure mode of broken heater wires resulting in sparking, arcing and fires are totally unacceptable. Therefore, resistive wire-based heaters are not used in the operating room today.
Since the mid 1990's, a number of inventors have tried unsuccessfully to make effective and safe heated mattresses for operating room use, using flexible, sheet-like electric resistance heaters. The sheet-like heaters have been shown to be more effective in warming the patients because of the even heat production and generally do not cause arcing and sparking when they fail.
Some existing devices employ sheet-like heaters using a polymeric fabric that has been baked at high temperature until it becomes carbonized and is thus conductive of electricity. The carbonization process makes the fabric fragile, and therefore, it may be laminated between two layers of plastic film or fiber-reinforced plastic film for stability and strength. The lamination process results in a relatively stiff, although somewhat flexible, non-stretching, non-conforming heater. The metal foil bus bars are attached to the heater material with an “electrically conductive adhesive or bonding composition . . . ” and then encapsulated with polyurethane-coated nylon fabric. The result is a stiff and relatively inflexible bus bar.
Other sheet-like heaters found in some existing devices use a carbon-filled electrically conductive ink, printed on and laminated between two sheets of polyester film. The copper braid bus bars are “suspended” in the carbon-filled plastic and also laminated between the two sheets of polyester film. The resulting heater and bus bar assembly is relatively stiff, non-conforming and totally non-stretching. Because the heater is relatively stiff, a layer of foam, preferably greater than 1.5 inches thick (0.25-3 inches), can be placed between the heater and the patient. This thick layer of foam may pad the patient from the stiff heater, but it also introduces a thermal insulation between the heater and the patient, making the mattress ineffective for patient warming. Finally, the heater elements of this invention are similar to flat wires and are not “sheet-like.” Polyester film can be cut out of the large spaces between the individual heater elements in order to improve flexibility. With this design, it is impossible to produce even heat across the surface of the pad, as it would be with any wire heater for use in a warming pad. It is hot where the wire is located and cold in between the wires.
In other devices, the heater material is a carbon impregnated plastic film. The film contains >50% carbon by weight. The carbon-laden plastic film is relatively weak and non-elastic and therefore is extruded or laminated onto a woven fabric for stability and to prevent tearing. The metal film and woven wire bus bars are bonded to the conductive plastic with a conductive adhesive and then potted in a thick layer of plastic for durability and strength. The fabric-reinforced film heater is relatively flexible, but is not stretchable or elastic. The potted bus bars are relatively inflexible and totally non-stretchable. Such devices can include a thick layer of high-loft fibrous thermal insulation placed between the heater and the upper surface of the mattress/patient. This thermal insulation reduces the effectiveness of the mattress for patient warming.
Electrically conductive fabric made of carbon fibers has been used as heater material in therapeutic blankets. However, carbon fiber fabric has not been used for therapeutic mattresses. Carbon fiber fabric may be stabilized by laminating it between layers of plastic film in order to keep the “slippery” fiber bundles from shifting randomly and altering the conductivity and heat production. Additionally, the carbon fibers are known to fracture over time with repeated flexing, which also changes the conductivity. Fiber fracturing can be minimized by laminating the fabric between layers of plastic film. The stiffer the resultant laminate, the more protective of the fibers. However, stiff heaters are not optimal when used in therapeutic heating blankets and mattresses because they are opposite of localized pressure reduction. Finally, carbon fiber fabric is known to not heat evenly, often resulting in “hot spots.” Skin is fairly intolerant of heat and therefore the temperature of the applied heat from the mattress is preferably accurately and tightly controlled. If the temperature of the heater is not even, accurate control is impossible.
In summary, designs that incorporate electrically conductive fabric heaters are of necessity relatively stiff because of the need to be laminated between two layers of plastic film. These laminated heaters are somewhat flexible and can be deformed into a simple curve. However, they do not respond to point pressure applied to their surfaces and deform into three-dimensional compound curves resembling a half sphere without folding and wrinkling. This is because these laminates do not stretch. Stretching would desirably provide evenly distributed, non-wrinkling 3-dimensional deformation. Finally, these heaters all utilize bonding and laminating or potting of the bus bars to the heater material in order to assure a durable electrical connection attempting to avoid “hot” bus bar failures. The heaters become very inflexible and totally non-stretchable in the areas of the bus bars. Therefore, these laminated fabric heaters have limited utility for use in pressure-reducing therapeutic mattresses.
Conductive and semi-conductive films are often made into heaters by applying the film to a relatively non-stretchable fabric because the carbon-laden plastic film is relatively weak and inelastic and because even if the film did not tear while stretching, it would not return to its original planar shape when the deforming pressure is removed.
Another existing device includes an inflatable air mattress with a single air chamber and a heater incorporating a resistive wire heating element stretched across its upper surface. This mattress design may be suitable for home use, but the single chamber design is not maximally accommodating and is relatively unstable for surgical table use. The wire heating element is totally unsuitable for use in the operating room. Finally, the heater is attached to the mattress around its edges and, thus, would exhibit hammocking when deformed by the weight of a patient.
Maximal patient warming effectiveness is achieved by maximally accommodating the patient into the mattress. In other words, maximizing the contact area between the patient's skin and the heated surface of the mattress. The heater and the foam or air bladders of the mattress may be easily deformable to allow the patient to sink into the mattress. This accommodation maximizes the patients skin surface area in contact with the mattress and heater, which minimizes the pressure applied to any given point. It also maximizes the surface contact area for heat transfer and maximizes blood flow to the skin in contact with the heat for optimal heat transfer. The accommodation of the patient into the mattress may not be hindered by a stiff, non-conforming, non-stretching, hammocking heater. Additionally, the heater should be near the top surface of the mattress, in thermally conductive contact with the patient's skin, not buried beneath thick layers of foam or fibrous insulation.
Clearly, there is a need for conductive fabric heaters for use in therapeutic heated mattresses that are highly flexible, stretchable in at least one direction and durable without needing lamination to stabilize or protect the heater fabric. There is also a need for bus bar construction that does not result in thick, stiff, inflexible areas along the side edges of the heater. Then, maximally effective and safe therapeutic heated mattresses need to be designed using the stretchable, durable fabric heaters.
As known to those skilled in the art, modern surgical techniques typically employ radio frequency (RF) cautery to cut and coagulate bleeding encountered in performing surgical procedures. Every electrosurgical generator system may have an active electrode that is applied by the surgeon to the patient at the surgical site to perform surgery and an electrical return path from the patient back to the generator. The active electrode at the point of contact with the patient may be small in size to produce a high current density in order to produce a surgical effect of cutting or coagulating tissue. The return electrode, which carries the same current as the active electrode, may be large enough in effective surface area at the point of communication with the patient such that a low density current flows from the patient to the return electrode. If a relatively high current density is produced at the return electrode, the temperature of the patient's skin and underlying tissue will rise in this area and can result in a patient burn.
Return electrodes have evolved over the years from small 12×7-inch, flat stainless steel plates coated with a conductive gel that were placed under the patient's buttocks, thigh, shoulders, or any location where gravity could ensure adequate contact. The next development was flexible foam-backed electrodes. These flexible electrodes are about the same size as the stainless steel plates and are coated with a conductive polymer. They have an adhesive border so that they remain attached to the patient without the aid of gravity.
Described as early as 1938 and first introduced into the surgical market in 1960, capacitively coupled return electrodes offer an alternative to conductive return electrodes. Unlike conductive electrodes, which involve direct patient contact, a capacitively coupled electrode is placed close to, but not touching, the patient. It is separated from the patient by a dielectric barrier—that is, a layer of insulating material. This allows the electrode to form a capacitor with the patient. A capacitor is an electrical circuit element used to store a charge temporarily. In use, this type of electrode induces a current flow across the electrode-patient capacitor such that electricity is safely returned from the patient to the electrosurgical unit across a dielectric insulator layer, allowing the desired surgical effect at the surgical site.
A capacitively coupled return electrode consists of a single conductive plate, fabric or film that is encased in a dielectric material. The insulating material does not permit the charge to flow through the electrode to the patient. When placed in close proximity to each other, the conductive plate and the patient become capacitively coupled. Their separation is maintained by the electrode's insulating material, which forms a dielectric barrier between them. For example, a large flat sheet of conductive material that covers a portion of the operating table may be the electrode and the dielectric barrier may consist of plastic film, linens, cushions or other materials that may be placed between the patient and the electrode.
When the active electrode is applied at the surgical site, the electrosurgical unit induces an oscillating radio frequency (RF) voltage through the surgical site and between the patient and the return electrode's conductive plate. As this occurs, several events take place simultaneously. First, an electrical charge accumulates and diminishes in cycles, both on the surface of the patient overlapping the return electrode and on the electrode's capacitive plate, in equal and opposing polarities. Second, the dielectric material becomes polarized: an electrical charge will not move through it. Finally, as the electrical charge moves to and from the surface of the patient's skin, there is a loss of energy that produces a minimal amount of heat within the skin (as happens with a conductive return electrode).
If the dielectric is thin, meaning that the patient and the return electrode are close together—for example less than 2 mm—the capacitive coupling is very efficient. If the distance between the patient and the electrode increases, the efficiency of the coupling decreases. Therefore, minimizing the distance between the patient and the electrode may be desirable. The ability of this design to minimize the distance of both the heater and the grounding electrode from the patient may be particularly desirable with small pediatric patients who have minimal surface area contacting the support surface.
There is some concern that an unnoticed, accidental hole in the electrode's dielectric material could provide a conductive contact with the patient over a very small area, causing a large concentration of current to flow in a small area and to burn the patient. In some cases, thick layers of “self-sealing” gel material have been interposed between the electrode and the dielectric material to prevent a conductive pathway from occurring in the event of a hole in the dielectric material. The gel material is heavy and cumbersome.
Capacitive coupling electrodes generally have been mattress overlays, which are inconvenient, involving extra cleaning. Additionally, they are usually non-stretching conductive fabric—for example, woven nylon embedded into a heavy, cumbersome gel pad—which reduces the effectiveness of the pressure-reducing mattress of the surgical table. The conductive silver coating on the fabric electrode also diminishes radiolucency to x-rays, causing x-rays that are shot through the mattress to be grainy or distorted.
The location of the capacitive coupling grounding electrode under the patient is in direct competition for space with heated underbody warming pads and mattresses commonly used in surgery. Heated underbody warming pads and mattresses also work optimally when in close contact with the patient's skin. Therefore, both of these safety technologies may not perform optimally when used simultaneously as two separate devices since seemingly only one or the other can be optimally placed adjacent the patient's skin.